1. Field of the Invention
This invention relates to a technique of forming an image by correcting the position of forming a toner image on the transfer surface of an intermediate transfer body.
2. Description of the Related Art
In an image forming apparatus designed to transfer a toner image formed on the transfer surface of an intermediate transfer body onto a sheet of paper, the position where the toner image is formed on the transfer surface of the intermediate transfer body needs to be correct because it directly affects the position to which the toner image is to be transferred onto the sheet of paper (the image position on the page of the sheet) is actually transferred from the intermediate transfer body onto the sheet.
When forming an image by means of a plurality of toners of different colors (e.g., black (K), cyan (C), magenta (M) and yellow (Y)), it is necessary to make the positions to which the toner images of the different colors are transferred correct and appropriate relative to each other (color superimposition) because a so-called color shift (a degraded image) appears if the toner images of the different colors are displaced from each other on the surface to which the toner images are transferred as shown in FIG. 4 of the accompanying drawings.
Because of the importance of the positions for forming toner images on the transfer surface of an intermediate transfer body, techniques for correcting the positions for forming toner images on the transfer surface have been proposed (see, inter alia, Patent Document 1: Jpn. Pat. Appln. Laid-Open Publication No. 2003-333607).
The technique of the above-cited Patent Document 1 is for improving the accuracy of the positions for forming toner images on the transfer surface by detecting the quantity of displacement of the image forming position of the toner image of each of the colors (in the direction of moving a sheet of paper) on the transfer surface and regulating (correcting) the timing of forming a corresponding image on a photosensitive body according to the detected quantity.
Meanwhile, in the case of an image forming apparatus adapted to form an image on a long-sized sheet of paper (A3/LD, etc.) and also on a short-sized sheet of paper (A4/LT, etc), a long toner image is formed at a time on the transfer surface for a long-sized page if images of a plurality of pages are to be formed in an image forming operation, whereas two or more than two toner images are formed at a time on the transfer surface for so many short-sized pages if images of a plurality of pages are to be formed in an image forming operation typically as shown in FIG. 5. FIG. 6 illustrates an example of classification of sheet sizes for a group of sheet sizes for which toner images are formed simultaneously on an intermediate transfer body for two pages and a group of sheet sizes for which only a toner image is formed on an intermediate transfer body for a single page.
FIG. 7 illustrates a conventional method of correcting the positions for forming toner images on the transfer surface of an intermediate transfer body for short-sized sheets of paper and also for long-sized sheets of paper. The operation of correcting the positions for forming toner images starts with the toner image that corresponds to the first page and, when the position of the toner image of the first page is corrected, the operation proceeds to correct the position for forming the toner image that corresponds to the second page.
Referring to an upper part of FIG. 7, assume that the toner image (for the first sheet of paper) of the first page is transferred onto the transfer surface of an intermediate transfer body for a long-sized sheet of paper (e.g., A3 size sheet). When the toner image (for the second sheet of paper) of the second page is transferred onto the transfer surface, the position for forming the toner image is corrected by using a correction value for the second page that is different from the correction value used for the first page.
The correction value for the first page differs from the correction value for the second and subsequent pages probably because the abutment of the intermediate transfer belt cleaner and the timing of abutment for the secondary transfer for the second or some other page differ from those of the first page (difference of printing sequence). Since the pages that come after the second page follows the sequence same as that of the second page for the causes of color shift, the correction value that is used for the second page is also used for those pages.
As pointed out above, with the conventional method for correcting the color shift, only the correction value for long-sized sheets of paper such as A3 size sheets that are frequently consumed is defined in advance and generally utilized also when forming images on sheets of paper of other sizes. Toner images of two pages are formed at the same time on the transfer surface of an intermediate transfer body when forming images on short-sized sheets of paper (e.g. A4 size sheets) and the position of one of the toner images, or the toner image for the first page, is corrected and the applied correction value that is used for the first page is also used for the second page without correcting the correction value.
The transfer bias (primary transfer bias) that is applied when transferring a toner image (for a primary transfer) from the photosensitive surface of a photosensitive body onto an intermediate transfer body is common to both long-sized sheets and short-sized sheets of paper and always output in each cycle of operation from the time of starting an operation of forming a number of toner images on the transfer surface of the intermediate transfer body simultaneously to the time of ending the operation of forming the images on the transfer surface (to be referred to as image forming cycle hereinafter) (see FIG. 7).
However, when forming two toner images on the transfer surface at a time for so many short-sized sheets of paper, a blank space is produced between the toner images for preceding pages (the first sheet and the third sheet of A4 size in FIG. 8) and the toner images for the immediately succeeding respective pages (the second sheet and the fourth sheet of A4 size in FIG. 8) and no image is formed in the blank space separating any two consecutive toner images.
It is known that the intermediate transfer body and the photosensitive surface of the photosensitive body are adsorbed each other by electrostatic force. It is also known that the adsorptivity is affected by the type of the toner image to be transferred and the presence or absence of a formed toner image. Thus, when forming short-sized toner images, the behaviors of the toner images of each color fluctuate under the influence of electrostatic adsorption due to the blank space where no toner exists. Therefore, when forming toner images on the transfer surface of an intermediate transfer body for a plurality of pages at a time, the positions for forming the toner images that corresponds to the second and subsequent pages (or to the second and the fourth sheets of A4 size in FIG. 8) are adversely affected (to give rise to color shifts).
Additionally, when the length of a long-sized sheet and that of two short-sized sheets plus an inter-sheet gap are compared, the latter is longer than the former as a matter of course. In other words, a blank space follows the length of a long-sized sheet on the transfer surface of an intermediate transfer body. Thus, the behavior of the leading end of a toner image for a long-sized sheet (which corresponds to the second sheet of A3 size in FIG. 8) differs from that of the leading end of a toner image for a former page of a short-sized sheet (which corresponds to the first sheet or the third sheet of A4 size in FIG. 8).